Continuously variable transmissions, hereinafter called CVT, have a first pair of cone pulleys upon an input shaft and a second pair of cone pulleys upon an output shaft. Each pair of cone pulleys comprises a first pulley stationary in axial direction and a second pulley movable in axial direction, hereinafter called respectively primary pulleys and secondary pulleys. Between the pairs of cone pulleys extends a belt-drive organ such as a sliding band. The primary pulley and secondary pulley are each adjusted by a pressure medium. For that purpose an electronic control device controls the pressure level of the adjustment spaces of primary pulley and secondary pulley via the electromagnetic actuators and hydraulic valves.
DE-OS 42 34 103 has disclosed an apparatus for control of the pressure of the primary pulley. The device comprises an electromagnetically controlled primary valve and a hydraulic secondary valve. The primary valve is controlled by an electronic control device. In case of line disconnection of the electronic control device to the primary valve, the device passes to emergency running. The pressure level of the primary pulley is determined here by the secondary valve. The position of the secondary valve is, in turn, adjusted by a mechanical coupling of the primary pulley. The device has the disadvantage of not coping with the complete breakdown of the electronic control device.